Pulse code operated electronic range indicator



Sept 20, 1949- A. B. JACOBSEN PULSE CODE OPERATED ELECTRONIC RANGE INDICATOR 2 sheets-sheet 1 Filed May 2. 1945 INVENTOR. ANDREW B. JACOBSEN A TTQRNEY Sept- 20, m49., A. B. JAcoBsEN I 2,482,544

PULSE ACODE OPERATED ELECTRONIC RANGE INDICATOR Filed May 2, 1945 2 sheets-sheet 2 FHG-2 Afl VL B Mm D L Ik L L l y f L k k L E .k l L NGA- INVENTOR. ANDREW B. JACOBSEN BY f' Q/ Patented Sept. 20, 1949 PULSE CODE l-2,4sf:,544

orERA'rED ELEcTRoNI RANGE INDICATOR Y `Andrew B. Jacobsen, Somerville, Mass., assigner, `by mesne assignments, to the United States of A merica as represented by the Secretary of War Y Application May 2, 1945, Serial N0. 591,587

' l My invention` relates `inV general to radio circuits and more particularly to those circuits used in the reception and interpretation of transmissions from certain typesl of radio navigational aids. I

In the navigation of airplanes or ships, knowledge of the crafts location relative to fixed geographic or marine objects-such as mountains, shorelines, or reefs is of prime importance. Such information can be gainedwby a process of triangulation employing at least two stationary radio beacon transmitting stations, the position of which is marked on the navigators charts, and a radio receiving system.v withdirectional characteristics on board the .navigablecraft More recently systemshavebeen developed in which a single xed beacon station, upon re,- ceipt of an interrogating signal from a transmit.- ter in a ship or airplane, radiates a signal, which may be in the form of short pulses of electromagnetic energy in a predeterminable sequence or code characteristic of the particular emitter. A directional antenna'associated with the mobile receiver facilitates determination of the bearing of the fixed transmitter from the moving ship or plane. The pulse sequence identifies the particular radiating station, and the range or distance to the transmitter can be found by measurement of the time required for radio waves to travel between the transmitter and receiver. The information thus gained is sulcient to allow calculation of the moving crafts position. Accordingly, it is one object oi-my invention to provide means for distinguishing *between transmissions from separate fixed stations as described above according tof the coding of such transmission. Another object of my invention'is to provide a circuit which will give a direct indication ofthe range to the fixed transmitter.

Still another object is to provide circuits which will automatically Search for the-iixed 'transmitter over a predetermined range of several miles.

Briey, the invention utilizes a Ydelay line decoder from which positive pulsesA are derived-to gate a coincidence circuit. The coincidence circuit cannot operate unless signals in a predetermined sequence are applied to the decoder. Voltages Varying linearly with time are also applied to the coincidence circuit. An error signal is developed in the coincidence circuitif the linearly varying voltage does not "bracket thepulses derived from the delay line correctly. f This signal is proportional torange andcan be used with an indicating means.1 f f 3 claims. (C1. 25m-27) The operation of the invention will be more apparent upon reference to the following specication, claims, and drawings, in which:

Fig. 1 is a diagram of one embodiment of the present invention;

Fig. 2 is a compilation of voltage waveforms pres-ent at various points in the circuit;

Fig. 3 shows how an error signal is developed from incorrect bracketing in the coincidence circuit; and

Fig. 4 shows correct bracketing.

Referring now to Fig. 1, to terminal 5 is connected a delay line 6 of any appropriate type. Delay line '6 is terminated in resistance 1 which is equal to the characteristic impedance of the delay line.

Tap' is connected to grids I3 and I4 of a multigrid vacuumv tube I5, and to grids I6 and I1 of a similar vacuum tube I8. These vacuum tubes Aare part of a coincidence circuit. Tap 9 is connected vthrough blocking capacitor 20 and resistor 2l to grid 22 of tube I5, and also through blocking capacitor 24 and resistor 25 to grid 26 of tube I8. A third tap I0 isconnected through capacitor 28 and resistor 29 to grid 30 of vacu-` um tube I5, connection being made from the junction of capacitor 28 and resistor 29 to'resistor 3l and thence to grid 32 of vacuum tube I8. Anode 35 of tube I5 is connected to load resistor 3S which in turn is tied toa suitable source of positive potential, resistor 36 being bypassed by capacitor 31. Similarly anode 39 of tube I8 is connected through load resistor 40 to a source of positive voltage which may or may not be the Vsaine one which supplies tube I5. Resistor 40 is bypass-ed by capacitor 4I. Grid 43 of tube I5 andlgrid 44 of tube I8 are Vbiased positively by a suitable voltage source.

Tap Il on delay line 6 is connected to grid 46 of double triode 41, a grid-return resistor 48 being connected between grid 46 and ground. .Tap I2 on delay line 6 is connected to grid 5I of the other half of double triode 41, resistor 52 functioning as a grid-return resistor. Anodes 55 and 56 are tied together and connected to a source of steady positive voltage. Cathodes 51 and 58 are also tied together and connected to ground through resistor 60. Resistor 69 also serves as a. cathode resistor for tubes l5 and I8, since cathode 6I of tube I5 and cathode 62 of tube I8 are connected directly to cathodes 51 and 58 of tube 41.

The invention also includes certain circuits Awhich are known in the art,ancl hence are repre-f the setting of potentiometer 14 which is connectf ed between a point of positive potential vand ground.

The trailing edge of the rectangularr pulse output of multivibrator 1I is diierentiated and applied to the wide gate generator 15. The output of generator 15, a rectangular voltage pulse of' positive polarity, is impressed through resistors 16, 29, and 3l onto grids4 30 and 32 of coincidence tubes I5 and I8. The leading edge of this wide -r gate is used to. trigger sloping gate generator 18. This circuit generates a voltage pulse of which at least a portion varies substantially linearly with time. This gate is amplied in ra push-pull amplifier 19, the purpose of which is'to provide in addition to the original sloping gate a similar gate which is opposite in polarity. vOne gate is applied to grid 22 of tube 15 through resistors 80 and 2l, While the other gate is impressed through resistors 8l and 25 onto grid 26 of tube I8.

Differential amplifier 83 compares the potentials existing on anodes and 39 and amplifies any difcrencesbetween them, and velocity circuit 84 converts the output of amplifier 83 from periodic pulses into a steady `voltage which is substantially proportional to the amplitude of the pulses.` This voltage is fed tothe sloping gate generator., and also appears at output terminal 85. VTo this terminal can be connected an indicating device to permit observation of the amount of voltage being fed back to sloping gate generator 18. Under normal operating conditions terminal 5 has impressed on it the outputof a vradio receiver consisting of positive video signals. These signals represent the modulation envelope of radio frequency energy radiated-by a Xed radio beacon as has been described hereinabove. ArrivingA at lterminal 5 in the form of voltage pulsesy in a coded sequence, the videosignals pass down delay line 6, Which translates the time interval between successive pulses into a nite distance on the delay line as determined by its constants.

If, for example, the artificial line. 6 has a total delay timeof four microseconds and the incoming signal consists of three onev micrcsecond pulses spaced at `one microsecond intervals, when the second pulse has arrived at terminal 5, the rst pulse has travelled halfway down the line; after four microseconds have elapsed, the third pulse is on terminal 5, the second pulse is halfway Vdown the line and the rst pulse hasV completed its transit of the line. Therefore, taps 8, 9, and I9 may be connected-respectively at the beginning, the center, and the end of delay line 6 so that there Will be a time when the three taps are simultaneously raised in potential.

Delay line 16 is vterminated by resistor 1 in its characteristic impedanceso that reflections from the end of the line willnot. occur to cause operation of the circuit at Vthe wrongtime.

Clearly, the arrangement of taps 8, 9, and II! need not be as described inthe above example, but can be altered as desired to .correspond to a particular coding sequence. Also, the delay time of line 6 may be any suitable value.

Each of taps 8, 9, and I0 is connected to one or more grids in each of coincidence tubes I5 and I8.

5 The cathodes 6I and 62 of these tubes are maintained at a positive potential by current flowing through cathode resistor 50,.. Hence, before conduction can occur in either coincidence tube, the grids must be raised to potentials roughly of the .eI-'der of that on the coincidence tube cathodes.

Positive voltage pulses must exist at each of taps 8, 9, and I9 for this condition to obtain.

There is a possibility that pulses longer in duration than the delay time introduced by artificial line 6 may be passed through the radio receiver which precedes the circuit shown in Fig. 1 and be impressed on terminal 5. In this case when the initial portion of such a pulse reaches tap I9, succeeding portions of the pulse will be present at taps 8 and 9. The same result may be caused by a series of veryv closely spaced pulses or by a combination of long Vand short pulses. The effect of these conditions is the same as if separate short pulses in the correct' code sequence were on line 6. However, only in the latter case it is desired to have the coincidence circuit operate. Hence a disabling circuit is employed to render theY coincidence circuit inoperative-unless precisely the proper-signal applied.

In this circuit. a, doubletriode 41 is used in a cathode follower connection, plates 55 and 56 being tied together, as are cathodes 51 and 58. Grids 45 and 5l are brought respectively to taps II ad I2 which are on delay line 6 but displaced from taps 8,V 9, and IU. Cathode resistor 6i] also functions as a. cathode resistor for. coincidence tubes I5 and I8.. If Vtap Il or tap I2 or both areV raised in potential, the corresponding grid or grids in tube 41 will `also be Iraised. This increases the current through tube. 41 and hence through resistor 60,. The increased IR drop across this resistor so elevates the potential of cathodes'l and 62 that'an'increment of positive voltageY appearing at taps 8, 9, and IIJ and applied 45 to the grids of coincidence tubes I5 and I8 is effectively 'nullied preventing conduction in these tubes. vThus an Vadditional safeguard is provided against actuation of the circuit by signals of the Wrong sort;

Assuming that a coded signal for which taps 8, 9., and lll are set up existsv on delay line 6, the coincidence'tube disabling circuit .will be inoperative and positive `voltage pulses will appear simultaneously on'grids I3,!,4, 22', and 38 of tube I5 and'on grids I6', I1', 26,1'and 32 of tube I8.

` The presence ofthese pulses alone, however, is not suiicient to render thecoincidence tubes conductive. The 'addition' of another positive voltage pulse, the wide gate, whichis longer in duration than the pulses which make up the coded signal, is necessary. To obtain this wide gate, trigger pulses which areV generated externally to the circuit shown in Fig. 1 and which are separated by a predetermined time interval which is related to linear distance in one embodiment approximately ten miles, by the velocity of propagation of electromagnetic waves, are applied to terminal 10 and lmay be used to trigger o multivibrator 1I. The latter, however, is controlled by disabling gate generator V12 which generates a substantially rectangular voltage pulse initiated by atrigger applied .toterminal 13V and terminated after `a length of time determined by the setting of potentiometer 14;/ While this, pulse is im- '75 pressed on multivibrator 1 I, the latterisncapable the range triggeripulses on .terminalv1ll and withV an interrogatingsignal radiated from the mobile transmitter-, the Ileading Vedge of the pulse output-4of multivibrator 1| will be delayed beh-ind the master-trigger by a time equivalent to an integral increment of range. Potentiometer 14 is adjusted so` that thisrincrement is .approximately the range toma preselected beacon trans-V mitter. The relatively sharp leading edge present in the pulseyoutput of multivibrator 1| is then used to trigger wide gatev generator 15. A diferentiatorstage of .any desirable type rmay be inserted between multivibrator 1| and gate generator 15 to;v formV a pulseofshort duration from the leadingedge of the output of multivibratorY 1| in order to obtainmore positive triggering action. Y n l v l l Wide gate generator 15being thus actuated, produces a positive-going voltage pulse which is a predetermined'number of "milesflong That is, the duration ofthis pulse, or.gate,is such that an interrogating signal canltravel a certain number of miles, elicit a'response from a fixed navigational transmitter, and the response will return to the interrogator before the termination of the gate. The maximumdistance at which the iixed'transmitter can be located is limited by the gate duration. A Y

Sloping gate generator 18 is triggered simultaneously with wide gate generator 15, producing a voltage .pulse with a sloping trailing edge. The duration of this pulse is variable and is controlled bye. voltagefedinto generator 1-8 from velocity circuit 84'. A type of circuit known in the art as a phantastron is welladapted for use as the sloping gate generator and has been used as such inV one embodiment of my invention.

The pulse output of'gate 'generator 18 is applied to sloping gate, Aamplifier 19,' which, being ofthe push-pull type,'has an'outputtwo wave-V forms which ares'imilar but inverted with respect to each 'other'.V One is' substantially the same as the output of vsloping gate generator 18 while the other is an inversion of the rst. f One oi these. gates, for .example the positive-going one, is applied to grid 22 of Vcoincidence tube l5, while the other, which in this case will be the negative-going gate, is impressed on grid 26 ofv coincidence Ytube |8. These gates/are not necessary for conduction in the coincidence tubes; their purpose is to give automatic range search and indication.

Assuming now that coded pulses are being fed to delay line 6, and'triggerpulses, as described hereinbefore, appear on-terminals 1B -and 13, the operation ofthe present invention will-,be summarized. Having an approximate knowledge of his position witli'relation to a given Xed navigational transmitter, the navigator adjusts potentiometer 14 until the length of the pulse applied-tomultivibrator 1| by disabling `gate generator 12 corresponds to a distance equal to or less than twice his estimate of the range to the desired fixed transmitter. An interrogating signal is sent out by the ship or plane, and as this signal is transmitted, a master trigger is out irregularities and fluctuations.

' tiometer 14 aords coarse range indication, while applied top terminal 4'|3and a range triggerv is applied to terminal 10. Disabling gate generator 12 preventsmultivibrator 1| from being triggered fora lengthVv of time determined by the setting of potentiometer 14. During this period, the in-` terrogating pulse has reached the Xed transmitter and a coded signal has been sent back inreply. Ivf` potentiometer 1|!v has been set properly, the disabling pulse applied to multivibrator12 will terminate shortly before the coded signal reaches thenaircraftor ship. Assuming this tobe the case, multivibratorl 1| is triggered by the :nextfra'nge trigger to arrive at terminal 10 after removal of the disabling pulse. Multivibrator 1|. thenr operatesand triggers oli wide gate generator 415in/nich Vin* turn actuates sloping` gate generator 18.

`Providing that potentiometer 1=4 has been set correctly, aperiod Vof time of the order of a few microseconds later the `coded signal will arrive at terminal 5, pass down delay line E `and appear at the proper grids of coincidence tubes I`5 and|8 beforerthe wide. gate has terminated. When pulses derived from the coded signal and the wide gate are coexistent on the grids of tubes V|'5fand AI8, conduction will occur, the amount of conduction lin each tube dependingupon the instantaneous. valueeof the sloping gates atthe time of conduction. A portion` vof one. of these gates increases substantially linearly with time, while a corresponding portion of the other decreases in the same way, theY circuit being so designed that atone point the gates are equal in potential. If conduction takes place in coincidence' tubes l5. and I8 prior or subsequent to this point, one 'tube will conduct more heavily than thev other.

Current ,flow inthe coincidence tubes causes a voltage drop in`load resistors 36 and 4U, the amount ofdrop. in each being proportional to theganodecurrent in the corresponding .tube and hence to the VYtime phase of the coded signal pulsesfrelative :tothe sloping gate. SinceV current 'flows in tubes l5 and I8 in short pulses, capacitors 31anol 4| are provided to smooth the voltage drop across resistors 3B and 48. These capacitorsY are charged when tubes I5 and i8 conductand discharge through resistors 3&5 and 40 when the coincidence tubes are non-conductive.

The voltage variations at anodes `35 and 39 are applied to differential amplier 83, in which the two voltages are compared. If a diierence exists. between the two, it is amplified and applied to velocity circuit 84 which further lters The voltage output. oi?v velocity circuit 84 appearing at terminal 85 is proportional to distance, and indieating meanscan be attached thereto and calibrated in terms of range. The setting of poteni-lne indication can be obtained from terminal 85 as described above.

The range Voltage is also fed back to sloping gate generator 18, acting on it in such a way as to move the sloping portion of the gate until the point of equal conduction coincides with the coded signal pulses. This. does n'ot reduce Ythe ne range indication to zero, because a voltage must be ymaintained at terminal 85 to hold the sloping Vgate at the'proper position.

In order to clarify and summarize the time relationships between the voltage waveforms that have been mentioned hereinbefore in connection 75 with various portions of the circuit, reference is 7 now-'made to-Fi"g.'2. Curves- A\ through Fieacli: represent al voltager variation with time; It i's-tov be noted that these curves' are drawn tothesame timescale", but notY necessarily to the same voltage scale; These curves are' periodic in naturel and-'slightly more thanl one cycleis'fshownin each. Curve A represents an interrogatingsignal radi ated by` a mobile transmitter. CurveAB isa; coded pulsesequence returned from a fixedbeaconl transmitter. It is assumed; here that thef se quence consists of three evenly spaced pulses; Curve C represents the beacon pulses asV applied to the various grids of" the coincidence tubes at' the timeof conduction. Curve D is a series of range trigger pulses spacedY at regular intervals. Curve E is amaster trigger pulse. Both themas'-I ter trigger pulse and one of the range trigger pulses are shown coincident with the interrogatingpulse shown in curve A. Curve F is-the output of the disabling gate generator,v` consisting ofa' rectangular pulse with a variable" trailing edge: Curve G is the output ofthe multivibrator which is actuated by the range trigger pulses. The ree lationship between-curves D, F, and G shows-how the disabling gate generator is used" to prevent triggering of the-wide gate and sloping gate generators until Vthe beacon reply has almostreached the mobileY receiver. Curve Hr shows thewide gate' which is generatedsimultaneously with curveG'. Curves J and K showthe sloping gates as theyA areapplied to the coincidence tubes.

Fig. 3 is anv enlargement of thesloping gates', exaggerated intime scale, which have been super'- imposed and are'shown with the gate pulse de= rived from the beacon reply. In'tliis'instancethe, sloping gates are not properly positioned with respect to the gate pulse and an errorsignal` is derived from the coincidencecircuitj. y Ashasbeen described hereinbefore, this error signal is ap-l pliedA to thesloping gate generator so as to' re:- position-thesloping gates and thiseffect' is shown i'n-Fig; 4 wherein4 the sloping gates properly'bra'oket the gate pulse so as to equalize conduction iny the coincidence tubes.

Thus it can be seen thatr my invention Will' decode a signal from'a navigational transmitter and give aldirectvindcation of the distance between such a` transmitter and a craft' which is to be navigated by it'.

Itis to. be emphasized thatwliat has beendescribed herein is only apreferred. embodiment of theA invention and I claim allr modifications. and adaptations therein' as may fall fairly'withinthe spirit and scope of the. hereinafter. appended' claims. l

What I claim isz' 1; In a radio circuit',v an articial delayl line,v means for applying voltage' pulses to said delay line, a plurality of multigrid vacuum tubes, means. for connectingthe grids of said multigrid'vacuum tubes to said delayv line so that when. voltage pulsesV in a predetermined sequence pass. down said delay line said grids will be simultaneously raised inA potential, a first. rectangular pulse generator, meansifor supplying to said rlrstY rectangular pulse generator trigger pulses spaced' at pre.- determined time intervals, a secondrectangular pulse generator, means for. triggeringsaidsecond rectangular pulse generator,.means vfor applying the output of said second rectangular pulse genagregar@ eratforftofsaid first rectangular pulse generator't'- prevent triggeringr of said rstgenerator during ay predeterminable period of time, a third rec'- tangular` pulse: generaton rneansfor causing the' output of said rst rectangular pulse generatortoftrigger'said thirdA rectangular pulse generator, means for applying the output of said thirdrectangularpulse generator' to at leastV one grid1 in each of saidV multigrid vacuum tubes, av fourth' pulse generator of whichatv leastva*portionl oft'he output isa substantially'linear function with respect to time, 'means'V forcausingVA the outputoff said' tldirdpuls'e Vgenerator to trigger said fourth. pulse generator, means for` inverting at least a portiorroft the output of said fourthA pulse generator; means'for applying' the invertedvoltage to atleast one of said grids' of" one of saidi muitigridl vacuum tubes, means for applyingl a furtherpor tion of the-output' 'of said fourthjpulse' generator to atleast: oneigridof anotherlof said multigridf vacuum tubes, load means; connected" to each anode ofsaid vacuum tubes, adiflerentiall ampli-y iier; means for applying voltages developed'acrossl saidV load; meansl to said differentialy amplifier, and 'means' for feeding back the output of said'A diierential amplier to said fourth v pulse generator whereby the time duration of? the output of said'V fourth pulse generator is altered4 in ac-A oord'ance-therewi'th.

2i, Ina radio circuit,A a pulsesequence discriminator,A means foriapplyi'ng voltage pulses to said pulse sequence discriminatorg'a plurality ofmu1 tigrid` vacuum tubes,.meansfoi applying the output of said: pulse sequencedi'scriminator tothegrids of said multigrid vacuum tubes,.a rstpulse generatorhaving anl output'atleast a portion of which varies substantially linearly withl respect to time, means for inverting at: least' aportion of the output of said rst pulse generator; means for applying said. inverted voltage to' at least one ofv the grids of'one of'jsaid multigrid vacuum-tubes,v means. for applying a furtherportionof the output of'said pulse generator'tov atleast one'grid ofA another of said multigrid-l vacuum tubes, a second pulsev generator, and means for utilizing' the output' ofsaidsecond pulse" generator toactuate said' rst pulse generator.

3; Apparatus in accordance withA claim 2l wherein there isprovided'in additiona thirdipulse oov Number Name Date `1,924,174: Wolf.; ...1.7 Aug.v 29, 1933I 2,089,639 ABedford Aug. 10, 1937 2,211,942 White Y Aug.- 20, 1940 lIronman PAT-Emis@ Number- Country Date` 

